Changing disk access operations to reduce servo control impact in a multiple actuator drive

ABSTRACT

In a disk drive apparatus, a first time period is determined, during which a first head driven by a first actuator will be performing a first disk access operation. A second time period is determined, during which a second head driven by a second actuator will be performing a second disk access operation. The first and second actuators are independently movable such that the first and second disk access operations are capable of being performed in parallel. If it is determined that the second disk access operation will impact servo control of the first disk access operation, at least one of the first and second disk access operations is changed to reduce the impact to the servo control of the first disk access operation.

SUMMARY

The present disclosure is directed to changing disk access operations toreduce servo control impact in a multiple actuator drive. In oneembodiment, a first time period is determined, during which a first headdriven by a first actuator will be performing a first disk accessoperation. A second time period is determined, during which a secondhead driven by a second actuator will be performing a second disk accessoperation. The first and second actuators are independently movable suchthat the first and second disk access operations are capable of beingperformed in parallel. If it is determined that the second disk accessoperation will impact servo control of the first disk access operation,at least one of the first and second disk access operations is changedto reduce the impact to the servo control of the first disk accessoperation.

These and other features and aspects of various embodiments may beunderstood in view of the following detailed discussion and accompanyingdrawings.

BRIEF DESCRIPTION OF THE DRAWINGS

The discussion below makes reference to the following figures, whereinthe same reference number may be used to identify the similar/samecomponent in multiple figures.

FIG. 1 is a diagram of an apparatus according to an example embodiment;

FIG. 2 is a block diagram showing disturbance and susceptibility datastructures according to an example embodiment;

FIGS. 3, 4, 5, 6, and 7 are block diagrams showing scheduling ofactuator operations using data structures according to exampleembodiments;

FIG. 8 is a block diagram of a disk drive apparatus according to anexample embodiment; and

FIG. 9 is a flowchart of a method according to an example embodiment.

DETAILED DESCRIPTION

The present disclosure generally relates to data storage devices thatutilize magnetic storage media, e.g., hard disk drives (HDDs).Additional HDD features described herein, generally described as“parallelism” architectures are seen as a way to improve HDD performancemeasures such as IOPS and latency. Generally, parallelism architecturesutilize multiple read/write heads in parallel. Such parallelism canincrease the rate of input/output operations (IOPS) and thereby speed upcertain operations. For example, the data read from two heads can becombined together into a single stream, thereby doubling the throughputrate of data sent to the host. In other examples, different heads canservice different read or write requests at the same time, therebyreducing overall latency, e.g., for random data access requests.

In embodiments described below, a hard disk drive includes multipleheads driven by the same or different actuators that can read from orwrite to one or more disks simultaneously. This may include separate andindependent reading/writing, such as heads that are servicing differentread/write requests. This may also include separate and dependentreading/writing, e.g., where parts of a single stream of data are beinghandled by different heads at the same time. The parallelismarchitectures is extended to other components that operate in the HDD,including system controllers, servo controllers, read/write channels,host interfaces, caches, etc.

In an HDD with multiple actuators, the movement of one of the actuatorscan interfere with the operation of the other actuator(s). The forceexerted by an actuator can cause mechanical disturbance/vibration thataffects other actuators. The greatest forces intentionally exertedduring normal operation include forces applied during seek accelerationand deceleration. These forces are most likely to impact the ability totrack settle and track follow on the other actuator(s). For certainoperations such as settling and following operations while writing, theconstraints are tighter than other operations (e.g., reading) due to thepossibility of destroying previously written data due to mistracking. Ifthese operations are disturbed significantly, then the write operationwill be delayed or suspended resulting in degraded performance.

In embodiments described below, scheduling decisions of actuators istranslated into time regions of potential disturbance to otheractuator(s) and disturbance susceptibility. Scheduling on each actuatoruses the disturbance factors of the other actuator(s) to determine theoptimal operation to schedule and its corresponding seek behavior. Inthis way, cross-actuator disturbance can be minimized resulting in anincrease in performance for some operations, e.g., write operations.

In FIG. 1, a diagram illustrates an apparatus 100 with parallelismfeatures according to example embodiments. The apparatus 100 includes atleast one magnetic disk 102 driven by a spindle motor 104. A head 106(also referred to as a read/write head, read head, write head, recordinghead, etc.) is held over a first surface 102 a of the disk 102 by an arm108. An actuator 114 moves (e.g., rotates) the arm 108 to place the head106 over different tracks on the disk 102. In one embodiment, the headincludes a read transducer 110 and/or a write transducer 112. The readtransducer 110 provides a signal in response to changing magnetic fieldson the disk 102, and is coupled to a controller (not shown) where theseparate signals are independently processed. The write transducer 112receives signals from the controller and converts them to magneticfields that change magnetic orientations of regions on the disk 102.

The apparatus 100 includes a second head 116 supported by a second arm118. The second head 116 is held over a second surface 102 b of the disk102 and actuator 114 causes the second arm 118 to move to differenttracks on the disk 102. The arm 118 may move together with arm 108, orthe arms 108, 118 may move independently (as indicated by dashed line onactuator 114 indicating a split actuator). In either configuration, thearms 108, 118 rotate around the same axis. The head 116 also includesread and/or write transducers 120. The transducers 120 are capable ofreading from and/or writing to disk surface 102 b simultaneously withone or both of read/write transducers 110, 112 that access disk surface102 a.

In another embodiment, the apparatus 100 includes a third head 126supported by a third arm 128. The third head 126 (and its associatedactuation hardware) may be included instead of or in addition to thesecond head 116. The third head 126 is held over the first surface 102 aof the disk 102 as a second actuator 124 causes the third arm 118 tomove to different tracks on the disk 102. The arm 128 and actuator 124move independently of arm 108 and actuator 114. The head 126 includesread and/or write transducers 130. The transducers 130 are capable ofreading from and/or writing to disk surface 102 a simultaneously withtransducers 110, 112 of first head 106.

In the examples shown in FIG. 1, more than one disk 102 may be used, andthe actuators 114, 124 may be coupled to additional arms and heads thataccess some or all of the additional disk surfaces. In this context,“accessing” generally refers to activating a read or write transducerand coupling the transducer to a read/write channel. Independentlymovable heads that utilize a split actuator 114 may generallysimultaneously access different surfaces, e.g., heads 106 and 116 accessdifferent surfaces 102 a, 102 b at the same time. Independently movableheads that utilize non-coaxial actuators 114, 124 may access the samesurface at the same time, e.g., heads 106 and 126 may both accesssurface 102 a at the same time, as well as accessing different surfacesat the same time.

One or more controllers 132 are coupled to the respective actuators 114,124 and control movement of the actuators 114, 124. The controllers 132may include systems on a chip that perform such operations as servocontrol, encoding and decoding of data written to and read from the disk102, queuing and formatting host commands, etc. As pertains to thediscussion below, the one or more controllers 132 have the ability toexecute multiple media read/write operations concurrently.

Seek acceleration and deceleration of one actuator potentially disturbsoperations on other actuator(s). The amount of disturbance may be uniquefor each actuator and vary based on the rate of acceleration anddeceleration as well as other factors such as radial position. Anoperation may be susceptible to disturbance. The amount ofsusceptibility is related to the phase of the operation, such as seeksettle and write track follow. Susceptibility may also vary peractuator, radial position, and other factors.

Potential disturbance and disturbance susceptibility may or may not bemutually exclusive. In FIG. 2, a block diagram illustrates trackingstructures 200, 210 used to reduce the effects of disturbance accordingto an example embodiment. It is assumed that disturbance and disturbancesusceptibility are not mutually exclusive so they are tracked separatelyin the individual tracking structures 200, 210.

Each cell of the tracking structures is associated with an explicit time(e.g., system time T) or inherent time (e.g., current time plus N). Thedata in the structures 200, 210 are populated from the current time 220to a finite amount of time in the future (e.g., 50 ms) to reflect allscheduled operations on the actuator. Each entry contains a scaled valuethat represents the potential disturbance (in structure 200) orsusceptibility (in structure 210) for an amount of time. The scaledvalues are one-byte hexadecimal values in this example, the value ofwhich is determined based on factors noted above (e.g., radial position,type of operation, relative location of actuator, etc.). Note that thedescriptive entries in the top row of the structures 200, 210 areintended to show example operations and are optional, e.g., not requiredto be stored with the structures 200, 210. An alternative to the scaledvalues shown in the structures 200, 210 is an index to be used for atable look-up. In other embodiments, disturbance and/or susceptibilitycould be computed based on formulas. Tables could provide the inputs tothe formulas. Or instead of tables, a set of values/formulas could beused with a corresponding time duration. For example, disturbance couldbe represented by formula A for the next X ms, formula B for the next yms.

In the illustrated example, Actuator N schedules a write operation attime period 215 while currently write track following at time 211. Afterdetermining the rate of the seek operation, this decision is translatedinto the tracking structures. Based on when the seek operation isscheduled to begin, seek acceleration and deceleration regions aretranslated into scaled disturbance values 202, 204 while the timing ofwrite seek settle and track follow are translated into scaledsusceptibility values 212, 214.

Each actuator translates scheduling decisions in this manner, and thetranslated data may be viewed by one or more schedulers that are usedfor servo control of the multiple actuators. The tracking structuredescribes expected timing; however there are instances when actualbehavior is not as expected due to seek variance and other exceptionconditions. When these exception conditions occur, they can be ignoredresulting in some miscalculations (see below) and poor schedulingdecisions or the tracking structures can be adjusted upon detecting theexceptions to adjust the expected timing (e.g., shift values left orright in the structures to reduce or eliminate overlay, alter seekparameters to minimize interference).

Scheduling is performed for operations on each actuator. One goal ofscheduling is to optimize performance (e.g., to minimize the time toservice operations). Other factors may also be considered, such aspower. When scheduling an operation, the access time of an operation iscomputed. For example, access time may be computed as seek time pluslatency time. This result may be adjusted for other factors, such as theprobability of the seek not completing in time.

What is proposed is that the computed access time is further adjusted byan offset time. The offset time may include one or both of the followingterms: disturbance time and susceptibility time. The disturbance time isthe computed time penalty on operations on other actuator(s). Thesusceptibility time is the computed time penalty on this operation dueto disturbance caused by other actuator(s). The susceptibility trackingstructures of the other actuator(s) are used for this computation. Thedisturbance forces of the operation under consideration are overlaidonto the susceptibility structures to determine the time penaltyassociated with the possibility that this operation causes a missedrevolution (e.g., late settle or off-track fault).

In FIGS. 3 and 4, block diagrams show how scheduling can be performedfor Actuator M. In these figures, structure 210 includes thesusceptibility data from Actuator N shown FIG. 2, which is overlaid witha structure 300 that includes potential disturbance expected whenscheduling an operation for Actuator M. As shown in FIG. 3, theoperation under consideration is a write with a seek operation, the seekcan begin as soon as time 302. The potential disturbance due to the seekis translated and overlaid onto the susceptibility of Actuator N. Inthis case, Actuator N may get disturbed by Actuator M between times 302and 304, while the other disturbance forces do not pose a risk.

In order to quantify the potential disturbance on Actuator N, adisturbance time may be defined. The disturbance time is a function ofthe value of the susceptibility scaling factor and the disturbancescaling factor. One possible method to determine disturbance time T_(d)is shown in Equation (1) below, where p_(d) is probability of adisturbance, t_(rev) is revolution time, S is susceptibility value fromstructure 210, D is disturbance value from structure 300, and norm is anormalization value (e.g., maximum size of word used to store S and D).Applying this to the example in FIG. 3, this results in T_(d)=70 h*40h/FFFFh*t_(rev). If the structures 210, 300 store index values ratherthan scaled values, then another possible method to find the disturbancetime is shown in Equation (2) below, where F_(d) is a disturbance lookupfunction, and i_(S) and i_(D) are respective susceptibility anddisturbance indices.T _(d) =p _(d) t _(rev)=((S*D)/norm)t _(rev)  (1)T _(d,lookup) =F _(d)(i _(S) ,i _(D))  (2)

If there exists more than one entry per time region that may beimpacted, then the probability calculation should take into account allentries. Probability of a disturbance for Z regions is calculated asshown in Equation (3) below, where p₁, p₂, . . . , p_(Z) are respectiveprobabilities for regions 1 to Z. These computations are valid for theprobability of a single fault. The probability of more faults may alsobe calculated, however there is diminishing value in doing so as theprobability of multiple faults is significantly lower.p _(d)=1−(1−p ₁)×(1−p ₂)× . . . ×(1−p _(Z))  (3)

For the operation shown in FIG. 3, the scheduler can consider delayingseek initiation, slowing the seek operation, and/or modifying the seekstate transitions to minimize or eliminate the disturbance time.Examples of slowing the seek include limiting maximum acceleration anddeceleration and modifying the seek current profile. Examples ofmodifying the seek state transitions include: pulse width modulation(PWM) on/off, rotational vibration feed-forward (RVFF) on/off, otheradaptive feed-forward subsystem on/off, and single to dual stageactuation switching. Dual stage actuation involves activating amicroactuator that is located near the read/write head to provide fineadjustment to the head's location. For these cases, the access timecomputation involving seek time and latency time is re-computed as theslower or delayed seek may have an impact on these times.

In FIG. 4, a block diagram shows the result of delaying the seek shownin FIG. 3. The seek has been moved later in time (to the right in thefigure) and now begins at time 303. This results in a disturbance timeof zero as there are no overlapping cells in structures 210, 300 withnon-zero values. The disturbance time may be scaled or adjusted basedupon other environmental or operating conditions, such as operationalvibration.

A concept similar to disturbance time is susceptibility time, which usesthe susceptibility tracking structures of the other actuator(s). Thesusceptibility of the operation under consideration is overlaid onto thedisturbance structures of schedule operations to determine the timepenalty associated with the possibility that this operation misses arevolution (e.g., late settle or off-track fault). An example ofdetermining a susceptibility time is shown in the block diagram of FIG.5.

This block diagram shows scheduling a seek operation for Actuator N. Theoperation under consideration is a write with a seek operation, as shownin structure 500. The seek can be initiated no earlier than time 504.The potential susceptibility is translated and overlaid onto thedisturbance susceptibility of Actuator N, which is shown in structure502. In this example, Actuator M may get disturbed by Actuator N betweentimes 506 and 508. The susceptibility time is a function of the value ofthe susceptibility scaling factor and the disturbance scaling factor.The calculation is done in the same or similar manner of disturbancetime shown above.

To make a scheduling decision, the expected access time plus disturbancetime plus susceptibility time is computed for operations that arepending. Other terms may or may not exist in this equation and anembodiment may only consider disturbance time or susceptibility time. Ifthe scheduling policy is to schedule a single command with the lowesttime penalty for the multi-actuator device, then the operation with thelowest computed time value is selected.

In this example of FIG. 5, one solution to reduce disturbance onactuator M is to slow down the seek operation. This solution is shown inFIG. 6. Note that while the seek still begins at time 504, theacceleration values are lower, resulting in the deceleration happeninglater, at time 600. Note that there is still some overlap between thestructures 500, 502 at time 600, however the level of disturbance (20 hin this case) may be acceptable in order to have the seek of Actuator Ncomplete in a target amount of time, e.g., without missing a revolution.In other cases, the acceleration may be reduced even more so thatdeceleration occurs at or beyond time 508.

Other scheduling policies may favor one actuator over another or maymake decisions based on selecting more than a singular command. In thesecases, the computed time is a primary input into the decision method.For example, in the operations shown in FIG. 5, it may be that theoperations performed by Actuator N have higher priority than theoperations being performed by Actuator M. This priority may be on aper-operation and/or per actuator. In either event, because Actuator Nhas higher priority, the seek action shown in structure 500 may proceedas planned. The tracking operation shown in structure 502 may be delayedas shown in FIG. 7 to avoid interference.

In FIG. 8, a block diagram illustrates a data storage drive 800 thatutilizes one or more actuators according to example embodiments. Theapparatus includes circuitry 802 such as one or more device controllers804 that process read and write commands and associated data from a hostdevice 806 via a host interface 807. The host interface 807 includescircuitry that enables electronic communications via standard busprotocols (e.g., SATA, SAS, PCI, etc.). The host device 806 may includeany electronic device that can be communicatively coupled to store andretrieve data from a data storage device, e.g., a computer, a server, astorage controller. The device controller 804 is coupled to one or moreread/write channels 808 that read from and write to surfaces of one ormore magnetic disks 810.

The read/write channels 808 generally convert data between the digitalsignals processed by the device controller 804 and the analog signalsconducted through two or more heads 812, 832 during read operations. Thetwo or more heads 812, 832 each may include respective read transducerscapable of concurrently reading the disk 810, e.g., from the samesurface or different surfaces. The read transducers may be configured toread in any mode, such as conventional single-track with single reader,or various TDMR modes like single track with multiple readers (MSMR) ormulti-track with multiple readers (TDMR-MT). The two or more heads 812,832 may also include respective write transducers that concurrentlywrite to the disk 810. The write transducers may be configured to writeusing a HAMR energy source, and may write in various trackconfigurations, such as conventional, SMR, and IMR.

The read/write channels 808 may include analog and digital circuitrysuch as digital-to-analog converters, analog-to-digital converters,detectors, timing-recovery units, error correction units, etc. Theread/write channels 808 coupled to the heads 812, 832 via interfacecircuitry 813 that may include preamplifiers, filters, etc. As shown inthe figure, the read/write channels 808 are capable of concurrentlyprocessing one of a plurality of data streams from the multiple heads812, 832.

In addition to processing user data, the read/write channels 808 readservo data from servo marks 814 on the magnetic disk 810 via theread/write heads 812, 832. The servo data are sent to one or more servocontrollers 816 that use the data to provide position control signals817 to one or more actuators, as represented by voice coil motors (VCMs)818. The VCM 818 rotates an arm 820 upon which the read/write heads 812are mounted in response to the control signals 817. The position controlsignals 817 may also be sent to microactuators (not shown) thatindividually control each of the heads 812, e.g., causing smalldisplacements at each read/write head.

The VCM 818 may be a stacked or split actuator, in which case two VCMparts are configured to independently rotate different arms about acommon axis 819. In such a case, other heads (not shown) will accessdata on the disks simultaneously with that of heads 812, and these otherheads may be coupled to circuitry 802 similar to illustrated head 832.In other embodiments, a second actuator, e.g., VCM 828, mayindependently and simultaneously rotate a second arm 830 about a secondaxis 829. Corresponding heads 832 may be rotated by the VCM 828 and mayoperate simultaneously with the heads 812 under commands from the one ormore servo controllers 816.

One or more schedulers 840 access a common data structure 842 thataccess a common data store 842. This data store 842 may includesusceptibility and disturbance structures as described above, orequivalents thereof. The schedulers 840 are operable by the controller804 (or another processor or subprocessor) to determining time periodsduring which the multiple disk access operations will be performed. Ifit is found that that a target disk access operation will impact servocontrol of the other access operations, the target disk access operationis changed to reduce the impact to the servo control of the other diskaccess operations. This changing may include delaying the targetoperation, slowing down or speeding up the target operation, etc. Insome cases, the target operation may have a high priority, in which casethe other affect operations may be changed, e.g., delayed, paused, etc.

In FIG. 9, a flowchart shows a method according to an example. Themethod involves determining 900 a first time period during which a firsthead driven by a first actuator will be performing a first disk accessoperation. A second time period is also determined 901, during which asecond head driven by a second actuator will be performing a second diskaccess operation. The first and second actuators are independentlymovable such that the first and second disk access operations arecapable of being performed in parallel. It is determined 902 that thesecond disk access operation will impact servo control of the first diskaccess operation. One or both of the first and second disk accessoperations may be changed 903 to reduce the impact to the servo controlof the first disk access operation. Optionally, the method may alsoinvolve determining 904 that the first disk access operation will impactservo control of the second disk access operation, and make changes 905to one or both operations to reduce impact to the servo control of thesecond disk access operation.

The various embodiments described above may be implemented usingcircuitry, firmware, and/or software modules that interact to provideparticular results. One of skill in the arts can readily implement suchdescribed functionality, either at a modular level or as a whole, usingknowledge generally known in the art. For example, the flowcharts andcontrol diagrams illustrated herein may be used to createcomputer-readable instructions/code for execution by a processor. Suchinstructions may be stored on a non-transitory computer-readable mediumand transferred to the processor for execution as is known in the art.The structures and procedures shown above are only a representativeexample of embodiments that can be used to provide the functionsdescribed hereinabove.

The foregoing description of the example embodiments has been presentedfor the purposes of illustration and description. It is not intended tobe exhaustive or to limit the embodiments to the precise form disclosed.Many modifications and variations are possible in light of the aboveteaching. Any or all features of the disclosed embodiments can beapplied individually or in any combination are not meant to be limiting,but purely illustrative. It is intended that the scope of the inventionbe limited not with this detailed description, but rather determined bythe claims appended hereto.

What is claimed is:
 1. A method, comprising: determining a first timeperiod during which a first head driven by a first actuator will beperforming a first disk access operation; determining a second timeperiod during which a second head driven by a second actuator will beperforming a second disk access operation, the first and secondactuators independently movable such that the first and second diskaccess operations are capable of being performed in parallel;determining that the second disk access operation will impact servocontrol of the first disk access operation; and changing at least one ofthe first and second disk access operations to reduce the impact to theservo control of the first disk access operation.
 2. The method of claim1, wherein the first disk access operation comprises one of trackfollowing and settling.
 3. The method of claim 1, wherein the seconddisk access operation comprises one of seek acceleration ordeceleration.
 4. The method of claim 1, wherein determining that thesecond disk access operation will impact servo control of the first diskaccess operation comprises: forming first and second tracking structuresindexed from a current time to a future time, entries of the structureseach containing a value representing one of potential disturbance orsusceptibility for an associated time index; and comparing correspondingentries of the first and second tracking structures at a same futuretime to determine that a first potential disturbance of the firsttracking structure overlaps a first susceptibility of the secondtracking structure.
 5. The method of claim 1, wherein changing thesecond disk access operation comprises changing a start time of at leastone of the first and second disk operation.
 6. The method of claim 1,wherein changing the second disk access operation comprises changing oneof an acceleration and deceleration of the second actuator during thesecond disk access operation.
 7. The method of claim 1, furthercomprising determining first and second priorities of the first andsecond disk access operations, wherein the second disk access operationis changed to reduce the impact to the servo control of the first diskaccess operation based on whether a delay in the second disk accessoperation is acceptable in view of the second priority relative to thefirst priority.
 8. The method of claim 7, wherein if the delay in thesecond disk access operation is unacceptable in view of the secondpriority relative to the first priority, the method further comprisingchanging the first disk access operation instead of the second diskaccess operation.
 9. The method of claim 1, wherein changing the seconddisk access operation comprises: determining an offset time thatindicates a penalty for an access time of the second disk accessoperation due to the impact of the servo control on the first diskaccess operation; adding the offset time to the access time of thesecond disk access operation; and scheduling the second disk accessoperation according to the access time.
 10. An apparatus comprising:interface circuitry operable to communicate with: first and secondactuators; and first and second heads independently movable over a diskvia the respective first and second actuators such that first and seconddisk access operations are capable of being performed in parallel by therespective first and second heads; and a controller coupled to theinterface circuitry and operable to perform: determining first andsecond time periods during which the respective first and second diskaccess operations will be performed; determining that the second diskaccess operation will impact servo control of the first disk accessoperation; and changing at least one of the first and second disk accessoperations to reduce the impact to the servo control of the first diskaccess operation.
 11. The apparatus of claim 10, wherein the first diskaccess operation comprises one of track following and settling.
 12. Theapparatus of claim 10, wherein the second disk access operationcomprises one of seek acceleration or deceleration.
 13. The apparatus ofclaim 10, wherein determining that the second disk access operation willimpact servo control of the first disk access operation comprises:forming first and second tracking structures indexed from a current timeto a future time, entries of the structures each containing a valuerepresenting one of potential disturbance or susceptibility for anassociated time index; and comparing corresponding entries of the firstand second tracking structures at a same future time to determine that afirst potential disturbance of the first tracking structure overlaps afirst susceptibility of the second tracking structure.
 14. The apparatusof claim 10, wherein changing the second disk access operation compriseschanging a start time of at least one of the first and second diskoperations.
 15. The apparatus of claim 10, wherein changing the seconddisk access operation comprises changing one of an acceleration anddeceleration of the second actuator during the second disk accessoperation.
 16. The apparatus of claim 10, wherein the controller isfurther operable to determine first and second priorities of the firstand second disk access operations, wherein the second disk accessoperation is changed to reduce the impact to the servo control of thefirst disk access operation based on whether a delay in the second diskaccess operation is acceptable in view of the second priority relativeto the first priority.
 17. The apparatus of claim 16, wherein if thedelay in the second disk access operation is unacceptable in view of thesecond priority relative to the first priority, the controller isfurther operable to change the first disk access operation instead ofthe second disk access operation.
 18. The apparatus of claim 10, whereinchanging the second disk access operation comprises: determining anoffset time that indicates a penalty for an access time of the seconddisk access operation due to the impact of the servo control on thefirst disk access operation; adding the offset time to the access timeof the second disk access operation; and scheduling the second diskaccess operation according to the access time.
 19. An apparatus,comprising: at least one disk; first and second actuators; first andsecond heads independently movable over the at least one disk via therespective first and second actuators such that first and second diskaccess operations are capable of being performed in parallel by therespective first and second heads; and a controller coupled to the firstand second actuators and operable to perform: determining first andsecond time periods during which the respective first and second diskaccess operations will be performed; determining that the second diskaccess operation will impact servo control of the first disk accessoperation; and changing at least one of the first and second disk accessoperations to reduce the impact to the servo control of the first diskaccess operation.